Apparatus for mixing a pigment dispersion into a polyamide melt

ABSTRACT

Process and apparatus for the uniform mixing of a polyamide melt with a pigment dispersed in a liquid medium soluble in the polyamide wherein the melt is divided, immediately after introduction of the liquid pigment dispersion as a thin liquid stream, into planar segments or displaced layers having a maximum thickness of about 1,500 microns, preferably 20 to 850 microns, using a paddle wheel stirring mechanism at the inlet end of an elongated mixing vessel tapering radially inwardly down to its outlet or discharge end. This invention generally relates to a process for continuously mixing a pigment dispersion into a high molecular weight organic polymer which is in the molten state, for example nylon 6 (polycaprolactam), nylon 6,6 (polyhexamethylene adipamide) or other polyamides, particularly at the conclusion of a continuous polymerization or polycondensation process.

United States Patent [15] 3,704,863 Meyer et al. [45 1 Dec. 5, 1972'[54] APPARATUS FOR MIXING A PIGMENT 3,425,667 2/ 1969 Schutte et al..259/7 E E E INTO A POLY E FOREIGN PATENTS OR APPLICATIONS [72]Inventors: Ernst Meyenoberbmcl eland; 314,058 1/1934 Italy ..259/8Wolfgang Grimm, Erlenbach; Klaus Prim ry Examiner-Walter A. Scheel 2???g g? g Lmhar" Assistant ExaminerPhilip R. Coe

Sc en 0 emany Attorney-Johnston, Root, OKeefi'e, Keil, Thompson [73]Assignee: Akzona Incorporated, Asheville, & shlll'tlefi' 57 ABSTRACT[22] Filed: April 9, 1970 l Process and apparatus for the uniform mixingof a PP 31,443 polyamide melt with a pigment dispersed in a liquidmedium soluble in the polyamide wherein the melt is Related ApplicationDam divided, immediately after introduction of the liquid [62] Divisionof Ser. No. 777,370, Nov. 20, 1968, pigment dispersion as a thin liquidstream, into planar Pat. No. 3,560,430. segments or displaced layershaving a maximum thickness of about 1,500 microns, preferably 20 to [30]Foreign Application Priority Data 850 microns, using a paddle wheelstirring mechanism at the inlet end of an elongated mixmg vesseltapering NOV. 21, 1967 Germany ..P 16 94 348.9 radially inwardly down toits outnet or discharge end.

52 us. (:1 ..259/8 This invention generally relates to a process for 51Int Cl. ..B01f 7/16 Iinuously mixing a Pigment dispersion into a high[58] Field of Search 259/7 8 23 24 molecular weight organic polymerwhich is in the mol ten state, for example nylon 6 (polycaprolactam),nylon 6,6 (polyhexamethylene adipamide) or other [56] References cuedpolyamides, particularly at the conclusion of a con UNITED STATES P NTtinuous polymerization or polycondensation process.

2,787,447 4/1957 Crawford ..259/8 7 Claims, 1 Drawing Figure APPARATUSFOR MIXING A PIGMENT DISPERSION INTO A POLYAMIDE MELT The presentapplication is a division of application Ser. No. 777,370, now US. Pat.No. 3,560,430 filed Nov. 20, 1968.

The mixing of a pigment dispersion into the initially prepared melt of ahigh molecular weight organic polymers, e.g., in conjunction with acontinuous polycondensation or polymerization process, has not yet beensolved in a satisfactory manner. It is true that a process for theproduction of delustered or colored structures of polyamides,polyesters, polyurethanes or polyureas is known, in which a mixture of afinely divided pigment with a silicone is incorporated into the meltafter it leaves the polymerization apparatus and the melt is thenshaped, e. g., into films, filaments or the like. However, a pigmentdispersion in which the dispersing agent or liquid vehicle is a siliconeoil, or generally any liquid which is insoluble in the polymer, isunsuitable for the manufacture of spun-dyed yarns and the like. Thepigment incorporated into ultrafme silicone oil droplets also exists inisolated form in the solidified polymer; that is to say, it is notactually incorporated into the polymer. In particular, the extraction ofmonomers from apolymer such as nylon 6, for example, results in awashing out of those ultrafine silicone oil droplets having pigmentincorporated therein which are disposed at or in the vicinity of thesurface. This danger also exists to a considerable extent with asubsequent hydrosetting treatment. Furthermore, the droplets of siliconeoil or any other liquid insoluble in the polymer have a disturbingeffect, for example, when drawing or stretching the spun fibers,filaments or yarns.

When working with polyamide melts, it has therefore proved to benecessary to use a dispersing agent which is soluble in the polyamide;otherwise, a satisfactory incorporation of the pigment particles intothe polymer is not achieved. In addition, it has also been shown to beadvantageous for the dispersingagent to be soluble in water, since thepossibility then exists for washing out thisagent, for example, whilesimultaneously extracting the monomers from nylon 6.

The mixing of pigment dispersions into a prepared polyamide melt howeverpresents considerable difficulties where the liquid dispersing agent isnecessarily soluble in the melt, because immediately after theintroduction of the dispersion into the melt, the soluble liquiddiffuses into the melt at a very high rate of speed. A so-called dryingout of the pigment dispersion thus takes place, and this leads to theindividual pigment particles caking together or agglomerating intoaggregates of considerable size which then makes the melt unsuitable forspinning or extrusion through a small orifice. Consequently, it has alsobeen a usual practice, when manufacturing fiber-forming linearpolyamides which are to be spun-dyed or delustered, to produce the meltsfor the dye-spinning in an autoclave, the pigment suspension alwaysbeing admixed at the start of the polycondensation process. Thisbatch-wise production of the melt is a considerable disadvantage ascompared to the more desirable continuous polycondensation in which theresulting melt can be immediately directed to the continuous spinningoperation with or without intermediate granulation of the melt.

melts.

In the copending application of Meyer et al, Ser. No. 682,967, nowabandoned, filed Nov. 14,1967, which is a continuation-in-part of theparent application, Ser. No. 546,597, now abandoned, filed May 2, 1 966,a process for the production of spun-dyed filaments or yarns frompigmented polyamides is described, in

which butyrolactam is used as the essential dispersing agent. Thisdispersing agent is .very suitable for the preparation of pigmentdispersions which are to be added to polyamides before or at the startof the polycondensation process; nevertheless, the difficultiesdescribed in the preceding paragraph occur when they are added afterpolycondensation to the prepared One object of the invention accordinglyis the provision of a processby which it is possible for a pigmentdispersion, the liquid vehicle or dispersing medium of which is solublein the polyamide, to be mixed into the initially prepared melt of apolyamide without resulting in the occurrence of those agglomerates ofpigment particles which are quite troublesome and which severelyinterfere with the spinning process, e.g., by clogging the spinningorifices.

Another object of the invention is to provide an apparatus for uniformlyadmixing the polyamide melt with the polyamide dispersion in acontinuous manner such that the pigment becomes uniformly distributedthroughout the melt quite rapidly before the liquid dispersing agentdiffuses into the polyamide in which it is highly soluble.

These and other objects and advantages of the invention are explained ingreater detail in the following specification.

lt has now been found, in accordance with the invention, that especiallyadvantageous results are achieved in a process for continuously mixing apigment dispersion into a polyamide melt, where: the liquid medium usedto obtain a homogeneous distribution of the pigment in the initialdispersion issoluble in the polyamide, if the polyamide melt is dividedimmediately after introduction of a thin liquid stream of the pigmentdispersion into planar segments or layers having a layer thickness ofless than 1,500 microns, the planar segments being displaced relativelyto one another, and continuously withdrawing the resulting mixture in adirection substantially perpendicular to the planar segments. Thedivision of the polyamide melt into planar segments can be easilyaccomplished in a substantially cylindrical mixing zone by means ofrotating shearing edges arranged closely adjacent to a flat circularsurface at the inlet side of the mixing zone while continuouslyintroducing the polyamide and a thin stream of the pigment dispersionthrough the flat surface into the mixing zone. The plane or crosssection of the mixing zone at which the melt is divided into planarsegments or layers is thus advantageously spaced not more than about1,500 microns from the point. of entry of the pigment dispersion,preferably so as to provide a layer thickness of at least 20 microns upto about 850 microns.

It was also found that the peripheral speed of the shearing edges, e.g.,as provided by stirrer blades, at the ends of the blades or the outerperiphery of the layer of melt should not be lower than approximately0.4 m/sec and not more than approximately 3.1 m/sec; the range between alower peripheral speed of about 1.2 rn/sec and an upper peripheral speedof about 2.3 m/sec has proven to be most desirable. Depending on thetemperature of the melt and the nature of the dispersing agent,individual lumps or aggregates of pigment are already formed atperipheral speeds which are below 1.0 m/sec, and these aggregates becomeso marked in ranges below about 0.4 m/sec that they have a highlydisturbing effect on any subsequent spinning process.

It is generally desirable to maintain the polyamide melt in the mixingvessel at a temperature of approximately 260 to 290 C., alwayssufficiently high to permit the desired mean flow velocity but belowthose temperatures capable of causing further polycondensation or damageto the melt or the pigment dispersion. In most cases, the pigmentdispersion is added to the melt in an amount not greater than about 20percent by weight, preferably about 1 to 10 percent by weight, depending upon the desired content of pigment in the final spun-dyed ordelustered product. Thus, the pigment content of the final product isgenerally anywhere from 0.01 to 10 percent, but usually up to aboutpercent and preferably from about 0.1 to 3 percent, by weight withreference to the melt. Butyrolactam or a mixture of butyrolactamcontaining a minor proportion of caprolactam is especially useful as theliquid dispersing agent for the pigment dispersion.

According to the prior art, it was to be expected that a very intensivestirring and mixing operation would be necessary to uniformly distributethe pigment in the melt. However, it has surprisingly been found thatthis is not the case but that the essential feature according to theinvention resides in the step of dividing the melt into very thin layersor planar segments with a thickness in the range of the aforementioneddimensions, andthat an intensive stirring operation is not necessary oreven desirable to produce this effect, provided that the condition ofthe layer thickness, on the one hand, and the stirring speed, on theother hand, are kept within the limits indicated. These are produced byeffecting a suitable stirring operation in a cross section of the mixingvessel which is so adapted to the melt throughput that the mean flowvelocity of the melt through said cross section is between 4 X and 7 X10' m/sec, advantageously between 5 X 10" and 6.5 X 10' m/sec, this meanflow velocity being measured in the axial direction, i.e., perpendicularto the cross-section of the mixing zone and in the axial direction ofthe melt flow in the mixing vessel.

Apparatus which is especially suitable for carrying out the process ofthe invention includes a mixing vessel which is radially symmetricalaround a central axis with the inlet end of the vessel being enclosed orcovered by a flat circular plate member or surface perpendicular to saidcentral axis and with the sides of the vessel tapering radially inwardlydown to the diameter of an outlet opening at the opposite end of thevessel. The tapered structure of the vessel is advantageouslypear-shaped, i.e., starting with a gradual reduction in the diameter ofthe vessel, e.g., at or just below the lower edges of stirring blades inthe upper mixing zone,

and then a more rapidly increasing reduction in diameter as the sides ofthe vessel extend downwardly to the discharge or outlet opening. In thismanner, the mean flow velocity of the melt in the upper mixing zone isfrom each other around the drive shaft. These blades should bepositioned at adistance of about 800 to 2,000 microns from the flatplate or. covering surface at the inlet end of the vessel, preferablysuch that the leading or upper edge of the blades are located at adistance from the flat circular surface of about 1,000 to 1,500 microns.These blades also extend parallel to the flat plate and radiallyoutwardly in close proximity to the inner circumferential wall surfaceof the mixing vessel.

The mixing vessel must also include inlet means for the injection of anarrow or thin liquid stream of the pigment dispersion, together withmeans to conduct and supply a metered amount of the pigment dispersioninto the vessel. The inlet means for this dispersion should be locatedon the outer one-third of the flat circular surface or cover plate butat a distance of at least about one-sixth of the length of a stirringblade from the inner circumferential wall surface of the mixing vessel.There may be one or more of such inlets adapted to rapidly inject thepigment dispersion as the melt is being cut or sheared into displacedlayers or segments, but good results are achieved with a single narrowbore or conduit of small diameter extending parallel to the axis ofrotation of the drive shaft.

lnlet means in the cover plate should also be provided for the polymermelt, preferably at one or more points also located in the outerone-third of the flat circular surface but at least about one-sixth of aradius inwardly from its outer periphery corresponding to the outerperipheral movement of the stirring blades. Again a single inlet for themelt is sufficient, preferably at a point diametrically opposed to theinlet for the dispersion. Finally, it is desirable to provide means tocontinuously withdraw a metered amount of the liquid mixture in thevessel through the outlet opening.

The circular cross section of the vessel at the height or location ofthe paddle wheel stirring blades is advantageously selected atapproximately 200 cm per ton of 24 hour daily throughput. The stirrer,which preferably comprises three to four blades, is operated at a speedof rotation of about 40 to 400 r.p.m., advantageously from to 300 r.p.m.(revolutions per minute). The pitch angle of the stirrer blades, asnoted above, is generally between 10 and 30, advantageously betweenabout 15 and 24.

It has been found that a particularly favorable range exists as regardsthe ratio by weight between the dispersing agent and pigment, this ratiobeing selected such that in carrying out the mixing of the stream ofpigment dispersion flowing onto the melt layer, the diffusion of thesoluble liquid dispersant into the melt and the resulting drying out ofthe suspension occurs only after the distribution of the pigment hasbeen sufficientlyeffected. Especially when using butyrolactam as avehicle or dispersing agent for the pigment, it was found that thefavorable range or proportion of pigment is about to 50 percent andadvantageously to 40 percent by weight with reference to the totalweight of the dispersion. With a lower proportion of pigment, thenecessary quantity of the dispersing agent and more especially thequantity of the supplied dispersion becomes too great, while the dangerof agglomeration is very greatly increased when the indicated limit isexceeded.

It is to be noted here that one must normally employ a very finelydivided pigment with a particle size not larger than about I [1,(micron), since particles greater than 1 p. are not considered to bepermissible at the time of preparing the initial pigment dispersion.When carrying out the process according to the invention as set forthhereinabove, it is found that the average pigment size likewise remainsbelow this limit. Agglomerates up to a size of about 8 to 10 p. areso'rarely encountered that they do not interfere with the spinningprocess;

It is true that with sufficiently fine distribution of the pigment andcareful mixing procedures, most of the pigments which are resistant topolycondensation temperatures can be introduced in the so-called VK tubeat the start of the condensation process. However, a change in colornecessitates a stoppage of at least several days, since the VK tube mustbe initially cleaned in a very tedious and time-consuming process.Moreover, with the addition of the pigment dispersion to the monomers orprecondensate batch before the polycondensation, it is only possible touse those pigments which are very resistant to condensation. With thepresent invention, however, there now becomes available a whole seriesof pigments with very desirable properties, such as fastness to lightand bleaching, illuminating power and the like, which can certainlysurvive the mixing and spinning process, whereas they are notsufficiently resistant to the polycondensation process.

For the first time, it has also become possible for these pigments to beadded continuously by means of the process according to the invention,since they satisfactorily survive the relatively short residence timesin the polyamide melt.

It has moreover surprisingly been established that the pigmentdispersion can be added cold, i.e., at about room temperature, withoutthe uniform or homogeneous distribution of the pigment in the melt beingnegatively influenced to any measurable degree.

The invention is also further explained with reference to theaccompanying drawing, the single F IGURE of which is a partly schematicrepresentation of the process and apparatus according to the invention.

The melt coming from a polymerization or melting vessel as indicated at1 passes through a heated pipe 2 into a mixing vessel 3 which tapersdownwardly in pearshaped form, into which the melt enters through anopening or bore 14 in the cover plate or plates 4 and 5. Arrangedimmediately beneath the lower cover plate 4 of the symmetrical vessel 3is a propeller-type or paddle blade stirrer consisting essentially ofthe pitched blades 6 which are radially mounted on the motor drivendrive shaft 7 so to revolve at a very small distance from the coverplate 4. The mixing vessel is heated, ad-

vantageously by means of a heating jacket 8 having conduits 9 throughwhich a suitable heating fluid can be passed for indirect heat exchange.

j The pigment dispersion which is supplied from a storage vessel 10 andwhich is to be admixed with the polymer passes through a gear wheelmetering pump 11 and pipe 12 to an opening or bore 13, which is likewisesituated in the cover plate or plates of the vessel 3, at approximatelythe same distance from the stirrer axis as the opening for the melt.These bores or inlet openings are preferably parallel to the rotatingaxis of the stirrer.

The revolving blades 6, the speed of which is adjusted to thethroughput, divides the'melt together with the pigment dispersionentering at 13 into thin layers or planar segments which have athickness smaller than 1,500 p. and advantageously smaller than 850 p.but at least 20 it. Surprisingly an extra-ordinarily fine and uniformpigment distribution is achieved by this simple expedient.

The melt is withdrawn from the mixing vessel 3 by way of a dischargepipe 15 and a metering pump 16 where it is advantageously first of allworked up into a granulated form, e.g., by a screw extruder l7 equippedfor granulation. Especially when the pigmented and granulated polyamidematerial is nylon 6 (polycaprolactam), the material is subjected to anaqueous extraction process forremoving the monomeric fractions orresidue, it also being possible at this point to simultaneously wash outthe dispersing agent, provided it is also soluble in water as well as inpolyamide.

The throughput of the polyamide melt as it proceeds from the inlet endof the mixing vessel 3 down to the outlet opening 18 can be easilyadjusted by means of the gear wheel metering pump 16. Due to thetapering side walls of the vessel, it will be apparent that the meltmoves downwardly or axially of the vessel most slowly in that zone ofthe mixer having the greater diameter, i.e., in and immediately adjacentto the rotating paddle blades 6. The mixing or distribution of thepigment in the polyamide melt takes place almost completely in thisupper mixing zone of greatest: diameter, provided that the peripheralspeed of the blades is at least about 500 times the axial mean flowvelocity of the melt. Due to the shearing forces applied by the rotatingblades, the melt and the added pigment dispersion also have a rotationaldirection of movement in the planar segment or layer as it is formed,the rotational mean flow velocity over a cross section of the layerbeing relatively difficult to determine although it is obviously muchless than the peripheral speed of the blades. Surprisingly, highshearing forces or an intensive mixing action is not at all essentialprovided that the layer thickness is maintained within the limitsdefined herein. Then, in the lower portion of the mixing vessel wherethe crosssectional diameter decreases radially inwardly, the axial meanflow velocity of each displaced or segmented layer of the melt graduallyincreases so that in effect each layer tends to become gradually thickerwithout any high degree of mixing between adjacent layers. Nevertheless,the distribution of the pigment remains quite uniform, not only in eachlayer but also throughout a continuous run extending over a long periodof time.

When carrying out the process according to the invention, it has thusbeen found that the most important feature of the process is to avoidagglomeration of pigment particles immediately after the introduction orinjection of the pigment dispersion as a thin liquid stream at the inletend of the mixing vessel. Where this is achieved, as is made possiblefor the first time by the process of the invention, any inhomogeneitieswhich still exist in the pigment distribution can be completely balancedout during the subsequent melting 'of the granulate, for example in anextruder, the fine distribution of the pigment particles beingmaintained.

The invention is further explained by the following examples which areintended to be illustrative only. As the mixing device in these examplesthere is used a vessel as shown in the drawing with a diameter of 152 mmat the upper inlet end where a four-bladed stirrer with a pitch angle ofthe stirrer blades of is so arranged that the upper or leading bladeedges move about 1,200 p. below the inlet point of the melt and alsothat of the pigment dispersion on the lower flat surface of the plate 4.Butyrolactam was employed in both examples as the dispersing agent whichis soluble in the polyamide.

The values for the pigment distribution produced in the melt whenworking in accordance with the examples are set out in Table l forExample 1 and in Table 2 for Example 2.

The figures indicated in the tables in respect of the size distributionof the pigment particles, including agglomerate particles, weredetermined by microscopic counting of 80 different microtomepreparations with a layer thickness of about 10 p. and with amagnification of 1:450. The surface investigated was 0.26 mm. A countwas made only of those particles having a size of more than 2 IL, itbeing understood that these generally represent agglomerates since theaverage individual particle size is less than 1 ;1..

EXAMPLE 1 EXAMPLE 2 The melt throughput is 277 g/min. The injectedpigment dispersion contains percent by weight of titanium dioxide and isintroduced into the vessel at a rate of 17.8 g/min., so that thepigmented polymer leaving the mixer contains 1.60 percent by weight oftitanium dioxide. The speed of rotation of the stirrer is 250 r.p.m. Thepigment count by particle size yields the values set forth in Table 2.

TABLE I Counted r.p.m. of particles Distribution of the particles 2 ,1,

in the size classifications: stirrer 2 p. per 2-4 p, 4-8 p B-l6 p 16-25p.

TABLE 2 Counted r.p.m. of particles Distribution of the particles 2 p.

1 in the size classifications:

stirrer 2 ,t per 2-4 14-8 1.1. 8-16 ,t 16-25 ,1.

250 146 l42=97.3% #ZJI:

THE INVENTION IS HEREBY CLAIMED AS FOLLOWS: I v g 1. Apparatus for thecontinuous mixing of a polyamide melt with a liquid pigment dispersionwhich comprises:

a mixing vessel which is radially symmetrical around a central axis, theinlet end of said vessel being closed by a flat circular surfaceperpendicular to said centralaxis and the sides of said vessel taperingradially inwardly down to the diameter of an outlet opening at theopposite end of said vessel, rotatably driven paddle wheel stirringmeans including a drive shaft with its axis of rotation coinciding withsaid central axis and a plurality of pitched blades mounted radially onsaid drive shaft with an angle of pitch ofv about 10 to 30, said bladesbeing positioned at a distance of about 800 to 2,000 microns from saidflat circular surface and extending parallel thereto radially outwardlyin close proximity to the'inner circumferential wall surface of saidmixing vessel;

inlet means on the outer one-third of said flat circular surface but ata distance of at least aboutonesixth of the length of a stirring bladefrom the inner circumferential wall surface of the vessel for theinjection of a narrow stream of said liquid polymer dispersion, togetherwith means to conduct and supply a metered amount of said dispersioninto said vessel; 7 separate inlet means in said flat circular surfaceto continuously supply the polyamide melt into said vessel; and means tocontinuously withdraw a metered amount of the liquid mixture in saidvesselthrough said outlet opening.

2. Apparatus as claimed in claim 1 wherein the distance from the leadingor upper edge of said stirring blades from said flat circular surface isabout 1,000 to 1,500 microns.

3. Apparatus as claimed in claim 1 wherein the separate inlet means forsaid polyamide melt is also located in the outer one-third of said flatcircular surface but at a distance of at least about one-sixth of thelength of a stirring blade from the inner circumferential wall surfaceof said vessel.

4. Apparatus as claimed in claim 3 wherein each of said inlet means isarranged to inject a fluid in a direction extending parallel to the axisof rotation of said stirring means.

5. Apparatus as claimed in claim 3 wherein said inlet means are arrangedin diametrically opposed positions in said flat circular surface.

2. Apparatus as claimed in claim 1 wherein the distance from the leadingor upper edge of said stirring blades from said flat circular surface isabout 1,000 to 1,500 microns.
 3. Apparatus as claimed in claim 1 whereinthe separate inlet means for said polyamide melt is also located in theouter one-third of said flat circular surface but at a distance of atleast about one-sixth of the length of a stirring blade from the innercircumferential wall surface of said vessel.
 4. Apparatus as claimed inclaim 3 wherein each of said inlet means is arranged to inject a fluidin a direction extending parallel to the axis of rotation of saidstirring means.
 5. Apparatus as claimed in claim 3 wherein said inletmeans are arranged in diametrically opposed positions in said flatcircular surface.
 6. Apparatus as claimed in claim 1 wherein thecross-sectional area of said vessel at the position of said stirringmeans has an area of approximately 200 cm2. per ton of 24 hours dailythroughput.
 7. Apparatus as claimed in claim 1 wherein said paddle wheelhas three to four symmetrically mounted stirring blades with an angle ofpitch of each blade of between about 15* and 24*.